New ABC core for the synthesis of nonsymmetric star molecules

Article abstract of DOI:10.1021/ol0528585

An ABC core has been synthesized, with two orthogonal protecting groups (PG). This new multifunctional unit allows building of star-shaped molecules with up to three different moieties. The synthesis of a star-shaped mesogen based on an oligobenzoate scaf

Full text of DOI:10.1021/ol0528585

ORGANIC
LETTERS
2006
Vol. 8, No. 4
721-723
New ABC Core for the Synthesis of
Nonsymmetric Star Molecules
Matthias Lehmann* and Michael Jahr
Institute of Chemistry, Chemnitz UniVersity of Technology, Strasse der Nationen 62,
09111 Chemnitz, Germany
Matthias.Lehmann@chemie.tu-chemnitz.de
Received November 25, 2005
ABSTRACT
An ABC core has been synthesized, with two orthogonal protecting groups (PG). This new multifunctional unit allows building of star-shaped
molecules with up to three different moieties. The synthesis of a star-shaped mesogen based on an oligobenzoate scaffold with three arms
of different lengths is demonstrated.
Dendritic and star-shaped molecules are recently of high
interest in materials science because these multiarm com-
pounds are applied for optoelectronic devices,1 for drug
delivery,2 in catalysis,2,3 and also in liquid crystal science.4,5
Many of these materials have a 3-fold functional core with
a 1,3,5-substituted benzene ring, most of them with a uniform
periphery. Modification of the lateral groups has either been
realized statistically, converting peripheral functional groups
with a mixture of reagents,6 or been more elaborated by using
readily available multifunctional cores connecting different
dendrons.7 In this way, Janus molecules have been reported
with new amphiphilic properties.8 More complex ABC star
systems have been synthesized in the series of heteroarm
triblock copolymers and were shown to assemble in various
columnar structures.9 Theoretical studies predict numerous
new morphologies in such materials, if the interaction and
size of the branches can be controlled.10 Low molar mass
star compounds related to the latter block copolymers are
not known.11 However, somewhat different molecules, the
so-called bolaamphiphiles, with three incompatible building
blocks have been shown to form many new mesophase
morphologies.12 The synthesis of star-shaped molecules with
three dissimilar scaffolds connected to the center is highly
interesting with respect to the control of their supramolecular
(1) (a) Meier, H.; Lehmann, M. In Encyclopedia of Nanoscience and
Nanotechnology; American Scientific Publishers: Stevenson Ranch, 2004;
Vol. 10, 95. (b) Pomarenko, S. A.; Kirchmeyer, S.; Elscher, A.; Huisman,
B.-H.; Karbach, A.; Drechsler, D. AdV. Funct. Mater. 2003, 13, 591. (c)
Percec, V.; Glodde, M.; Bera, T. K.; Miura, Y.; Shiyanovskaya, I.; Singer,
K. D.; Balagurusamy, V. S. K.; Heiney, P. A.; Schnell, I.; Rapp, A.; Spiess,
H. W.; Hudson, S. D.; Duan, H. Nature 2002, 417, 384.
(7) (a) Grayson, S. M.; Fre´chet, J. M. J. Chem. ReV. 2001, 101, 3819.
(b) Grayson, S. M.; Fre´chet, J. M. J. J. Am. Chem. Soc. 2000, 122, 10335.
(8) (a) Saez, I. M.; Goodby, J. W. J. Mater. Chem. 2005, 15, 26-40.
(b) Saez, I. M.; Goodby, J. W. Chem.-Eur. J. 2003, 9, 4869.
(9) (a) Hu¨cksta¨dt, H.; Go¨pfert, A.; Abetz, V. Macromol. Chem. Phys.
2000, 201, 296-307. (b) Sioula, S.; Hadjichristidis, N.; Thomas, E. L.
Macromolecules 1998, 31, 8429. (c) Wei, J.; Huang, J. Macromolecules
2005, 38, 1107.
(10) Tang, P.; Qiu, F.; Zhang, H.; Yang, Y. J. Phys. Chem. B 2004,
108, 8434.
(11) To the best of our knowledge, no star-shaped mesogens with three
different arms related to terblockcopolymer are known today, except
compounds synthesized in our group (see ref 13).
(2) For a review, see: Bosman, A. W.; Janssen, H. M.; Meijer, E. W.
Chem. ReV. 1999, 99, 1665.
(3) For a recent review, see: van Heerbeek, R.; Kamer, P. C. J.; van
Leeuwen, P. W. N. M.; Reek, J. N. H. Chem. ReV. 2002, 102, 3717.
(4) For a review on unconventional mesogens, see: (a) Tschierske, C.
Annu. Rep. Prog. Chem., Sect. C 2001, 97, 191. (b) Guillon, D. Struct.
Bonding 1999, 95, 42. (c) Tschierske, C. J. Mater. Chem. 1998, 8, 1485.
(5) Lehmann, M.; Gearba, R. I.; Koch, M. H. J.; Ivanov, D. Chem. Mater.
2004, 16, 374.
(6) (a) Barbera´, J.; Donnio, B.; Gehringer, L.; Guillon, D.; Marcos, M.;
Omenat, A.; Serrano, J. L. J. Mater. Chem. 2005, 15, 4093. (b) Mart´ın-
Rapu´n, R.; Marcos, M.; Omenat, A.; Serrano, J. L.; Luckhurst, G. R.;
Mainal, A. Chem. Mater. 2004, 16, 4969.
(12) Cheng, X.; Prehm, M.; Das, M. K.; Kain, J.; Baumeister, U.; Diele,
S.; Leine, D.; Blume, A.; Tschierske, C. J. Am. Chem. Soc. 2003, 125,
10977.
10.1021/ol0528585 CCC: $33.50
© 2006 American Chemical Society
Published on Web 01/25/2006

arrangement in columnar self-assemblies. Mesogens with
three differently substituted oligobenzoate arms have been
synthesized using a 2-fold benzyl-protected phloroglucinol.13
The second arm has been coupled by stoichiometric control
yielding only small amounts of target molecules. In a
different context, dendritic molecules based on aromatic ether
scaffolds with up to four arms have been synthesized;
however, arm components have been attached early in the
multistep synthesis, affording only low overall yields. Thus,
dendrons previously synthesized were lost during these
procedures.14 The drawbacks can be overcome with a strategy
using orthogonal protecting groups, prior to the coupling of
arm scaffolds. These protecting groups have to be introduced
and cleaved successively, without influencing each other or
the attached benzoate scaffold. An ABC moiety based on
phloroglucinol has been synthesized earlier;15 however, in
conditions where one of the applied protecting groups, i.e.,
the benzenesulfonyl group, deprotects, benzoate functions
cleave, too. Thus, for an efficient synthesis of nonsymmetric
star oligobenzoates, a new ABC phloroglucinol derivative
should be designed. Promising phenol protecting groups for
phloroglucinol are the benzyl and the tert-butyldimethylsilyl
group, which can be cleaved individually without effect upon
the benzoate units.16
direct 3-fold O-benzylation leads to a minimum of 10% of
an additional Friedel-Crafts C-benzylated product, which
can be removed only with difficulty and in small amounts
of material by chromatography.18 Recently, Kawamoto and
co-workers developed a synthetic route to a pure 3-fold
O-benzylated phloroglucinol, which can be applied for large-
scale production.19 Here, the electron-rich benzene ring of
phloroglucinol could be deactivated vs Friedel-Crafts reac-
tions by 3-fold O-acetylation. Subsequent ester cleavage in
the presence of benzyl chloride led to the analytically pure
1,3,5-trisbenzyloxybenzene as starting material for the
synthesis of 1.
The synthetic route toward ABC core 1, starting with
1,3,5-trisbenzyloxybenzene 2, is outlined in Scheme 1.
Scheme 1. Synthesis of ABC Building Block 1
To build up the target ABC core, 1 (Figure 1), as a key
compound for the preparation of star-shaped or dendritic
molecules with different arms or dendrons, either the
protecting groups have to be attached stepwise or a protecting
group has to be removed from a 3-fold protected phloro-
glucinol. The latter strategy turned out to be the most efficient
Selective monodebenzylation was first achieved by the
method of Curtis et al.17b A subsequent etherification with
tert-butyldimethylsilyl chloride afforded the silyl ether 4 in
excellent yields. In the following debenzylation step, basic
conditions have to be avoided to not cleave the just
introduced silyl ether function. Thus, cyclohexene is em-
ployed as a hydrogen donor,17c which permits the controlled
monodeprotection in good yields. If the reaction is stopped
at the appropriate time, the single second compound isolated
apart from the target molecule is the unreacted starting
material, which can be reused in the deprotection procedure.
Reaction times that are too long result in a mixture with the
additional 2-fold deprotected product.
Key compound 1 can be used to introduce three different
arms by subsequent selective esterification and deprotection
procedures (Scheme 2). The conditions are optimized for
benzoate scaffolds.5,20 Esterification of the phenolic hydroxy
group with 3,4,5-tridodecyloxybenzoic acid provides com-
pound 5 in excellent yields. The silyl ether function is cleaved
by tetra-n-butylammonium fluoride with a small amount of
acetic acid to not provoke a transesterification process by
Figure 1. New ABC building block; a phloroglucinol core with
2-fold orthogonal protected phenolic OH groups (Bn ) benzyl).
because selective coupling of one or two protecting groups
to phloroglucinol affords mixtures of compounds which are
difficult to separate. For 1,3,5-trisbenzyloxybenzene, various
selective monodeprotection procedures are known17 and thus
it has been chosen as starting material. However, it should
be noted that its synthesis is not straightforward because
(13) Lehmann, M.; Gearba, R. I.; Koch, M. J. H.; Ivanov, D. Mol. Cryst.
Liq. Cryst. 2004, 411, 397.
(14) (a) Peerlings, H. W. I.; Struijk, M. P.; Meijer, E. W. Chirality 1998,
10, 46. (b) Kremers, J. A.; Meijer, E. W. J. Org. Chem. 1994, 59, 4262.
(15) Wolff, S.; Hoffmann, H. M. R. Synthesis 1988, 760.
(16) Green, T. W.; Wuts, P. G. M. Protecting Groups in Organic
Synthesis, 3rd ed.; Wiley: New York, 1999.
(18) Deme, E. J. Org. Chem. 1976, 41, 3769.
(17) (a) Chow, H.-F.; Wang, Z.-Y.; Lau, Y.-F. Tetrahedron 1998, 54,
13813. (b) Curtis, W. D.; Stoddart, J. F.; Jones, G. H. J. Chem. Soc., Perkin
I, 1977, 7, 785. (c) Nithyanandhan, J.; Jayraman, N. J. Org. Chem. 2002,
67, 6282.
(19) Kawamoto, H.; Nakatsubo, F.; Murakami, K. Synth. Commun. 1996,
26, 531.
(20) Wang, Q. M.; Bruce, D. W. Angew. Chem., Int. Ed. Engl. 1997,
36, 150.
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Org. Lett., Vol. 8, No. 4, 2006

Scheme 2. Preparation of a Nonsymmetric Star-Shaped
Molecule 9 with Three Different Benzoate Arms Attached
to the Phloroglucinol Core
Figure 2. Star-shaped mesogen 9 with three benzoate arms of
different lengths.
of benzoate units.5 The remarkable difference in mesogenic
behavior compared to that of symmetric mesogens will be
published in detail elsewhere. The synthesis of unsymmetrical
compounds will be extended to chemically and physically
different arms and dendrons in future work.
In summary, the new ABC building block 1 opens up the
possibility to synthesize new nonsymmetric star-shaped or
dendritic compounds, where three joined arm components
may distinguish in size or function and consequently lead
to new functional tripodic materials.
generation of a phenolate anion. By this procedure, com-
pound 6 with a phenolic OH group is obtained in high yield
and purity. A second benzoate arm can then be attached,
and the final phenolic hydroxy group is deprotected in
quantitative yields by hydrogenation to obtain molecule 8.
Finally, the third benzoate arm R3COOH is coupled to the
center to give target molecule 9 (Figure 2). By this strategy,
three different arms can be built around a central benzene
core, in high overall yields of up to 40% (five steps).
The arms of the resulting star-shaped compound 9
distinguish only in the length of the benzoate scaffold.
Interestingly, molecules 9 self-assemble in a liquid crystal
phase with a clearing temperature at 50.6 °C (onset, 2nd
heating), which is essentially the same temperature as that
observed for the clearing of the columnar hexagonal phase
formed from the symmetric mesogen with the same number
Acknowledgment. We thank Prof. H. Meier, A. Oehlhof,
and Dr. N. Hanold for the elemental analysis and FD mass
spectra completed at the University of Mainz. The Deutsche
Forschungsgemeinschaft (DFG) and the Bundesministerium
fu¨r Bildung und Forschung (BMBF) are gratefully acknowl-
edged for their financial support.
Supporting Information Available: Detailed experi-
mental procedures and characterization data of all com-
pounds. This material is available free of charge via the
Internet at http://pubs.acs.org.
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